Method and apparatus for connecting two messaging systems having differing synchronizations one of which is message-based

ABSTRACT

A method for connecting a system utilizing message-based synchronization with an external system. The message-based system includes a plurality of nodes interchanging signals containing synchronization messages with information about the priority of the respective signal in the internal synchronization hierarchy of the system. For the external system to be able to utilize the message-based system as well as possible with respect to timing, synchronization status data in the internal format of the external system is transmitted from the interface node of the system utilizing message-based synchronization to the external system. The synchronization status data is converted from the message-based synchronization signature (18c) transmitted by the interface node in such a manner that the level in the internal synchronization hierarchy of the external system as indicated by the synchronization status transmittd to the external system rises or falls corresponding to the rise and, correspondingly, fall of a predetermined magnitude of the level indicated by the message-based synchronization signature.

This application claims benefit of international applicationPCT/FI95/00096, filed Feb. 23, 1995.

BACKGROUND OF THE INVENTION

The present invention relates to a method for connecting a systemutilizing message-based synchronization with an external system, and anetwork arrangement according to the preamble of the appended claim 8.

An external system typically uses a synchronization method differentfrom that of a system utilizing message-based synchronization, but inprinciple, an external system can also use message-basedsynchronization, as will be described later.

As used in the text below, the term node refers to a junction pointbetween transmission lines in a system. A node may be any device orequipment capable of affecting clock synchronization, such as abranching or cross-connection means.

Nodes in a system utilizing message-based synchronization areinterconnected by transmission lines which the nodes use for datatransmission. These lines also forward the clock frequency of thetransmitting party to the receiving party. Each node selects thefrequency of a signal from a neighbouring node, the frequency of its owninternal clock source or a frequency applied to the node via a separatesynchronization input from an external clock source as the source of itsown clock frequency. In order that all nodes in the system will operateat the same clock frequency, one usually attempts to make the system tosynchronize itself with a single clock source called a master source.All system nodes connected directly to the selected master source arethus synchronized with the master source, while nodes connected to thenodes adjacent to the master source but not directly connected to themaster source are synchronized with these adjacent nodes. Accordingly,each node at a greater distance from the master source synchronizesitself with a node one node spacing closer to the master source.

In order that the above-described synchronization hierarchy can beestablished within the system, the system nodes interchangesynchronization messages. These messages contain information by means ofwhich individual nodes are able to select a timing source. The systemnodes are prioritised and the system tends to synchronize itself withthe clock frequency of a node having the highest level of priority.Normally each priority level is assigned to a single system node.Synchronization messages normally contain information about the originof the clock frequency of the node transmitting the message and thepriority of the node as well as a value describing the quality of theclock signal. Accordingly, a neighbouring node clock frequency whichoriginates from a desired node and which is of the highest quality canbe selected by an individual node as the source of its own clockfrequency. At the system start-up each node selects its own internalclock source as the source of its clock frequency as it has not yetprocessed any incoming synchronization messages. After the node hasprocessed the first incoming synchronization messages, it selects theclock frequency of a neighbouring node having the highest level ofpriority as the source of its clock frequency. After all messages havebeen distributed over the system and the system has achieved a stablestate as far as synchronization is concerned, the system has beensynchronized hierarchically with the clock frequency of the mastersource.

FIG. 1 shows a system MS utilizing message-based synchronization in astable situation. Priorities assigned to the nodes are indicated bynumbers within the circles representing the nodes. The smaller thenumber, the higher the priority of the node. Synchronization messagestransmitted by a node n (n=1 . . . 6) are indicated by the referenceMSGn. Synchronization messages transmitted by different nodes usuallydiffer from each other and depend on the applied message-basedsynchronization method. The distribution of the clock frequency from themaster clock (node 1) to the other system nodes is illustrated by solidlines. Internodal connections drawn by broken lines are not used in anormal situation for system synchronization, but they are available inchange situations.

Message-based synchronization is based on a simple principle that theuser defines the synchronization hierarchy of the nodes by assigningeach node a dedicated signature indicating the hierarchical level of thenode and that the system synchronizes itself with the defined masterclock independently by utilizing, if required, all existing internodalconnections. If the connection to the master clock fails, and noalternative connection exists, or if the master clock fails, the systemsynchronizes itself with a node of the next highest level of hierarchy.Response to a change in synchronization takes place by messageinterchange between nodes.

Message-based synchronization methods of the type described above aredescribed e.g. in U.S. Pat. Nos. 2,986,723 and 4,837,850, which arereferred to for a more detailed description. Messages used in one priorart message-based synchronization method (SOMS) will be described moreclosely below with reference to FIGS. 2 and 3.

A system employing message-based synchronization forms a kind of closedsystem, at least as far as synchronization is concerned (often also withrespect to data transmission). This is problematic as there neverthelessexists a need to interconnect different types of networks and thus alsoa need for the interconnected systems to be able to make use of eachother also with respect to timing.

SUMMARY OF THE INVENTION

The object of the present invention is thus to provide a networkarrangement by means of which one or more external systems are able toutilize a system employing message-based synchronization also withrespect to timing as well as possible.

The idea of the invention is to change the synchronization status to betransmitted to an external system dynamically in such a manner that itcorresponds to the synchronization state of a system utilizingmessage-based synchronization. (Synchronization status reveals thequality or priority of the clock frequency of the source ofsynchronization in the internal synchronization hierarchy of a system.Synchronization status is given in a synchronization message or as aseparate bit/bit pattern in a position assigned to it in the framestructure of a signal.)

Due to the solution provided by the invention, an external system isable to effectively utilize a message-based system in itssynchronization, because it is aware of the synchronization state of themessage-based system. The use of the external system connection for theinternal synchronization of the message-based system is also facilitatedwhen the solution of the invention is utilized.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention and its preferred embodiments will bedescribed in more detail with reference to the examples shown in FIGS. 2to 6 of the attached drawings, in which;

FIG. 1 illustrates a system employing message-based synchronization whenthe system is in synchronization with the clock frequency of the mastersource;

FIG. 2 illustrates a network employing self-organizing master-slavesynchronization (SOMS) in its initial state;

FIG. 3 illustrates the network of FIG. 2 in a stable state;

FIG. 4a illustrates schematically the connection of a system utilizingmessage-based synchronization with an external system, when oneconnection between the systems is used for synchronization;

FIG. 4b illustrates schematically the connection of a system utilizingmessage-based synchronization with an external system, when twoconnections between the systems are used for synchronization;

FIG. 5 illustrates the signature formation according to the invention ina first embodiment of the invention; and

FIG. 6 illustrates the signature formation according to the invention ina second embodiment of the invention.

DETAILED DESCRIPTION

FIG. 2 illustrates a system employing self-organizing master-slavesynchronization (SOMS), which is a message-based synchronizing methodknown per se. In this specific case, the system comprises five nodes (ordevices) assigned SOMS addresses indicated by the reference numerals 1 .. . 5 according to their level of hierarchy. (The master node of thesystem has the smallest SOMS address.) The nodes interchange messagescontaining such SOMS addresses. In this way, the nodes are able toidentify each other by means of the address numbers and establish asynchronization hierarchy so that the whole network can synchronizeitself with the master node.

As mentioned above, messages transmitted continually in the network aredependent on the applied message-based synchronization method. Inaddition, the messages are specific for each transmitting node. In theSOMS network a synchronization message contains three different parts: aframe structure, signature and check sum. The SOMS signature is the mostimportant part of the SOMS message. It comprises three consecutivenumbers D1 to D3:

D1 is the origin of the synchronization frequency of a node transmittinga SOMS message, i.e. the SOMS address of a node appearing as a masternode to the transmitting node.

D2 is a parameter describing the quality of the connection, typically adistance to a node indicated by D1. The distance is given as the numberof intermediate nodes.

D3 is the SOMS address of a transmitting node.

Each node (or device) compares continuously incoming SOMS signatureswith each other and selects the smallest amongst them. In the signaturethe different parts D1, D2 and D3 are combined into a single number byplacing them in succession (D1D2D3) (for the sake of clarity, a dashwill be inserted between the different parts in the text below asfollows: D1-D2-D3). Accordingly, a primary criterion for the selectionof the smallest address is the SOMS address (D1) of a node appearing asthe master node to the preceding nodes, i.e. the node tends to besynchronized with a signal having a frequency originally derived from anode with the smallest possible address. In a stable situation, thewhole network is thus synchronized with the same master node (as themaster node of the whole network has the smallest SOMS address).

If two or more of the incoming signals are synchronized with the samemaster node, the one arriving over the shortest path (D2) is selected.The last criterion for selection is the SOMS address (D3) of the nodetransmitting the SOMS message, which is used for the selection if theincoming signals cannot be distinguished from each other in any otherway.

After the node has accepted one of the neighbouring nodes as its newsynchronization source on the basis of an incoming SOMS signature, ithas to regenerate its own SOMS signature. The new SOMS signature can bederived from the selected smallest SOMS signature as follows: the firstpart (D1) is left intact; the second part (D2) is incremented by one,and the third part (D3) is replaced with the node's own SOMS address.

Each node also has its own internal SOMS signature X-O-X, where X is theSOMS address of the node. If none of the incoming SOMS messages containsa signature smaller than the internal signature, the node uses its owninternal oscillator or possibly a separate synchronization input as thesource of clock frequency. Of course, the outbound SOMS message therebyemploys the internal SOMS signature.

The nodes transmit continuously SOMS messages in all directions in orderthat any changed data in the SOMS signatures is distributed as rapidlyas possible and that they know the current operating condition ofneighbouring nodes. The SOMS signatures cannot be compared with eachother until the incoming SOMS messages have been accepted and the SOMSsignatures have been extracted from the messages.

When the first SOMS message is received from a specific transmissionline, the SOMS signature contained therein is accepted immediately forcomparison, if the message is faultless. When the incoming transmissionline has an accepted SOMS signature and faultless messages containingthe same signature are received continuously, the situation remainsunchanged. If the SOMS message is found to be faulty, the current SOMSsignature is retained until three successive faulty SOMS messages havebeen received. At this stage, the old SOMS signature is no longeraccepted for comparison. Waiting for three successive SOMS messages aimsat eliminating temporary disturbances.

If no SOMS message is received from the line and there is no linefailure, the current SOMS signature is rejected only after a period oftime corresponding to three successive SOMS messages. If the line failstotally, the SOMS signature is rejected immediately. If no appropriateSOMS signature is available for comparison due to disturbances in theincoming signal, the SOMS signature of the transmission line isrejected. A constant-value signature where all parts (D1, D2, D3) havetheir maximum value (MAX-MAX-MAX) is thereby used in the comparison asthe SOMS signature of this incoming transmission line.

When a new changed SOMS signature is detected in an incoming SOMSmessage, it is accepted immediately for comparison, if the message isfaultless. In this way, there will be no unnecessary delays in networkchanges.

Initially, each node employs its own internal synchronization source,and transmits its own internal SOMS signature X-O-X to the other nodes.This signature is also compared with incoming SOMS signatures. If noneof the incoming signatures is smaller than the internal signature, thenode continues to use its own internal timing.

In FIG. 2, the SOMS network is shown in an initial state when none ofthe nodes (or devices) has yet processed any one of the incoming SOMSmessages. In all nodes, the highest priority is assigned to the internalSOMS signature of the node as no other signatures have yet beenprocessed. In FIG. 2, the SOMS signatures are indicated beside each nodeto which they are transmitted, and the selected signature is writtenwithin the frame (in the initial situation shown in FIG. 2 all nodesemploy their internal timing source). Lines used in synchronization aredrawn as a continuous line and standby lines are drawn as a broken line(in the initial situation shown in FIG. 2, all lines are standby lines).

When the nodes start to process the incoming SOMS messages, node 1retains the use of the internal timing, nodes 2 and 4 synchronizethemselves with node 1 on the basis of the signature 1-0-1, node 3 issynchronized with node 2 (2-0-2), and node 5 with node 3 (3-0-3). At thesame time, the nodes generate their own new SOMS signatures as describedabove and provide their outbound SOMS message with the new signature.The network in a stable situation is shown in FIG. 3. All the nodes havesynchronized with the master node 1 over the shortest possible path.

In order for an external system to be able to utilize the message-basedsystem described above with respect to synchronization (to use themessage-based system as a source of synchronization), an interface isprovided between the system utilizing message-based synchronization andthe external system, through which interface synchronization status datais transmitted to the external system, this data corresponding to thesynchronization state of the message-based system at a given moment.

FIGS. 4a and 4b illustrate schematically the connection of a network MS2utilizing message-based synchronization with an external system ES. Theexternal system may be, for instance, a local network and themessage-based system may be a long-distance trunk network, or theexternal system may be a dedicated network and the message-based systemmay be a public network maintained by a teleoperator. The nodes, whichare indicated by mere circles, except the master node indicated by thereference M and the interface nodes of the network MS2 by the referencesIN1 and IN2, are thus conventional telecommunication network elementsand the connections between them typically 2 Mbit/s PCM connectionsaccording to CCITT recommendations G.703 and G.704.

In the example of FIG. 4a, only one connection (continuous line) betweenthe systems is used for synchronization, whereas in the example of FIG.4b, both connections between the systems are used for synchronization(conversion is carried out in both interface nodes).

Two alternatives can be used in implementing the solution of theinvention, depending on which part of the interface node (IN1 or IN2)the conversion is carried out in:

1. The conversion can be carried out in the message-processing part ofthe interface unit of the node, the signature of the message-basedsystem transmitted to the interface units by the decision-making part ofthe control unit being converted into a form required by the externalsystem only in the interface unit.

2. The conversion can be carried out by the synchronizationdecision-making part of the control unit of the node, whereby it forms asynchronization signature in accordance with the external system at thesame time it forms the node's own outbound synchronization signature inaccordance with the message-based system.

These alternatives are illustrated in FIGS. 5 and 6, which show theelements relating to conversion in the interface node IN1 or IN2 of asystem using message-based synchronization.

FIG. 5 illustrates the conversion taking place in the interface unit IUn(n=1, 2, . . . ) of a node of a message-based system. The figure showstwo connections (A and B) coming into (and going out of) the device(node) of the system, the connection A connecting to the interface unitIU1 of the node and the connection B connecting to the interface unitIU2 of the node. The connection A is connected to a node included in thesystem, the signals received from the connection thus includingsynchronization messages, which a signal transmitting and receiving unit13 extracts and transmits further to a synchronization messagetransmitting and receiving part 16 connected thereto. Thesynchronization message transmitting and receiving part verifies thatthe message is faultless and transmits the message 18a further to thesynchronization decision-making part 20 of the device, located in thecentralized control unit CU of the node. The signal transmitting andreceiving unit 13 also monitors the quality of the signal and storesdata on it in a first fault database 24 located in the interface unit.The synchronization message transmitting and receiving part 16 alsoreceives the data on faults from the database 24. If the incoming signalA is not acceptable as regards synchronization, the synchronizationmessage transmitting and receiving part 16 forbids using it. This can bedone by a separate forbiddance or by setting the value of the incomingsignature as the highest possible, whereby, in practice, it is not usedfor synchronization in any case in the latter case.

The decision-making part 20 of the control unit CU performs thecomparison of the synchronization messages received from all theinterface units IUn communicating with the nodes of the message-basedsystem, and forms a synchronization priority list on the basis of them,this list being stored in a memory 21. In the highest level of thepriority list is the used clock source with the "best" synchronizationsignature according to the used message-based synchronization method.From this signature, the decision-making part 20 forms the node's ownoutbound signature 18c, which it transmits to the message transmittingand receiving parts of all the interface units. In the interface unitswhich are connected to the external system, as in the interface unit IU2in the example of FIG. 5, the conversion of synchronization statusaccording to the invention is carried out. To put it more accurately,this takes place in a synchronization message transmitting and receivingpart 17, which applies the data which are obtained as a result of theconversion and which represent the status of the external system to asignal transmitting and receiving part 14, which forms a signal to betransmitted to the external system.

FIG. 6 illustrates the other alternative of practical implementation:providing the synchronization signature/synchronization status of theexternal system in a centralized manner together with providing thesignatures of the message-based system. Also in this case, theconnection A is connected to the internal device of the system MS2, asignal received from the connection thus containing a synchronizationmessage. The signal receiving part 13 transmits the synchronizationmessage to an interface-specific synchronization message transmittingand receiving part 18 connected thereto, the output of which isconnected to the common decision-making part 20 of the node located inthe control unit CU. The receiving part 18 checks the message 18a andthen transmits it to the synchronization decision-making part 20, whichforms a synchronization priority list in the memory 21. The receivingpart 13 transmits the data on faults of the signal received from theconnection A directly to the centralized fault database 26 of thedevice, from which database the decision-making part 20 receives theinformation and is thus able to indicate the synchronization signatureof the signal A to be useless during faults.

The decision-making part 20 of the control unit CU performs thecomparison of the synchronization messages received from all theinterface units IUn communicating with the nodes of the message-basedsystem, and forms a synchronization priority list on the basis of them,this list being stored in the memory 21. In the highest level of thepriority list is the used clock source with the "best" synchronizationsignature according to the used message-based synchronization method.From this signature, the decision-making part 20 forms the node's ownoutbound signature 18c, which it transmits to the message transmittingand receiving parts of all those interface units (in the figure, onlythe interface unit IU1) which communicate with a node of themessage-based system. When the decision-making part has formed thesignature 18c, it performs the conversion of this signature in themanner according to the invention. After this, the decision-making parttransmits the signature/status 18d obtained as a result of theconversion to the synchronization message transmitting and receivingparts 18 of those interface units which communicate with the externalsystem (interface unit IU2 in the example of the figure).

The conversion can also be carried out on the basis of a message-basedsynchronization signature received from the selected source of timing,because the synchronization signature and the message-basedsynchronization signature transmitted by the node are interdependent.

It should also be noticed that the interface units of the node cantransmit synchronization messages/statuses of different types. In otherwords, the same node may communicate with several external systems, theinternal synchronization methods of which may be different amongthemselves.

The conversion of an outbound synchronization status is carried outeither in a decentralized manner in the interface units or in acentralized manner in the control unit. In the examples of FIGS. 5 and6, the fault database of the interfaces is located accordingly; when asignature is converted in the interface unit, the fault database is alsolocated in the interface unit, and when a signature is converted in thecontrol unit, the fault database is also located in the control unit.The practical applications are not, however, restricted to the solutionsaccording to the above-mentioned examples. The fault database can belocated for instance in a different unit than the one in which theconversion is carried out. On the other hand, the fault database canalso be located both in the interface units and the control unit. Inthis case, each interface is aware of its own faults, but they aregathered in a centralized manner also to the control unit. This is themost preferred manner of implementation, but the use of the idea of theinvention is however not restricted to any alternative of implementingthe database.

In the nodes according to FIGS. 5 and 6, it is also possible to convertsynchronization statuses of signals arriving from the external system,even if this type of conversion is not necessary as such, because it isnot necessary for a message-based system of an upper level to know thestatus data of an external system of a lower level. This type ofconversion taking place in the other direction is described in moredetail in a parallel Finnish Patent Application No. 940927. (The mannerof conversion (centralized or distributed to the interface units) ofoutbound signatures is in no way dependent on the processing of incomingsignatures, either. In other words, the incoming signatures can beconverted either in a decentralized manner in the interface units or ina centralized manner in the control unit, entirely independently of howthe conversion of the outbound signatures is carried out).

The signal to be transmitted to the external system ES is, for instance,a PCM connection according to CCITT recommendations G.703 and G.704, asstated earlier, or a signal according to recommendations G.708 andG.709, in which case an SDH network is concerned. In the following, theexemplifying situation is one where the external system is a systemutilizing LP synchronization known as such, the system being connectedto a system utilizing message-based synchronization by one or more 2Mbit/s PCM connections, and the system utilizing message-basedsynchronization is a SOMS network as described above.

LP synchronization (Loop Protected) is an expansion of independentmaster-slave synchronization (used, for instance, in some commerciallyavailable node devices of Nokia Telecommunications Oy. In independentmaster-slave synchronization, each node makes its own decisionconcerning its synchronization without obtaining any information fromthe outside which would support the decision-making. Since the nodesmake their own decisions on synchronization independently, each nodemust be provided with definitions of with which node it will synchronizeitself. These definitions are typically made in the form of the prioritylist mentioned above, a node thus selecting among the suitable incomingsignals the one having the highest priority, i.e. the one with thehighest position on the list, as its source of synchronization. If thissignal is interrupted or its quality weakens to such a degree that itcannot be accepted as the source of synchronization any more, the nodeselects the signal having the next highest priority from the list. Thepriority list must be constructed in such a manner that all the nodesincluded in it are located between the node concerned and the masternode, the synchronization thus spreading from the master node to thelower levels.

LP synchronization helps to maintain the desired timing in loop networksby using two status bits mcb (master control bit) and lcb (loop controlbit) in addition to the priority lists mentioned above, these statusbits being transmitted between network nodes. It is thus possible todefine also such sources to be included in the priority lists which arenot located between the node concerned and the master node. The firststatus bit mcb indicates whether the synchronization originates from themaster node of the network. The master node defined for the networktransmits this bit as a logical zero in its outbound signals, and theother nodes forward it further if they are synchronized with a signal inwhich the value of the mcb bit is zero. The second status bit lcbindicates whether the synchronization comprises a loop. Each networknode transmits this bit as a logical one in the direction with which ithas synchronized itself and as a logical zero in other directions.

Each node uses its own priority list when selecting the source ofsynchronization, but it also checks the mcb and lcb bits in addition tochecking the state of the signal before making a choice. A nodeprimarily tends to find a connection the clock frequency of whichoriginates from the master node of the network (mcb=0). If there areseveral of these connections, the one in the highest level of thepriority list is selected. If this type of connection cannot be found(due to a fault), the node selects a working connection which has thehighest position on the priority list in the normal manner. However, itis always required of the selected connection (the source of timing)that its timing is not in a loop (lcb=0), even if the actual signal weresuitable for synchronization in other respects.

Since LP synchronization is a synchronization method known as such, itwill not be described any further in this context. LP synchronizationhas been described in more detail, for instance in a Master's Thesis byJukka Kainulainen "Sanomapohjainen alistuva synkronointi digitaalisissateleverkoissa", Technical University, Faculty of Information Technology,Espoo, 1993. LP Synchronization has also been described for instance inFinnish Patent Application No. 931166, which is also referred to for amore detailed description.

The status bits lcb and mcb of the external system ES using LPsynchronization can be transmitted for instance in bits selected by theuser in the time slot TS0 (in which case bits 4-8 can be used in everyother frame, because in every other frame there is a frame alignmentsignal in the time slot TS0), or, for instance, in bits 7 and 8 when theuser selects the time slot TSN (N=1 . . . 31). In the latter case, therequired capacity must be taken from the capacity assigned to thepayload.

The table below shows one example of conversion, which is carried out onthe interface between the systems, i.e. in the manner described aboveeither in the synchronization message receiving and transmitting part ofthe interface node or in the decision-making part of the interface node.

    ______________________________________                                        SOMS signature t   mcb bit lcb bit                                            ______________________________________                                        t<100-0-0          0       0                                                  100-0-0≦t<500-0-0                                                                         1       0                                                  500-0-0≦t<MAX-0-0                                                                         0       1                                                  MAX-0-0≦t   1       1                                                  ______________________________________                                    

The table shows the SOMS signature t going out of a node and theoutbound mcb and lcb bits, which the message-based system formsautomatically for the signals of the external system when the node's ownoutbound SOMS signature has been formed. The figures (100, 500 and MAX)appearing as the conversion limits of the SOMS signature are mereexamples and only represent the mutual magnitude of the figures.

If the external system is, for instance, the system described in theabove-mentioned U.S. Pat. No. 4,837,850 instead of a system using LPsynchronization, the conversion can be carried out for instance in thefollowing way.

    ______________________________________                                        SOMS signature t                                                                              Clock message T                                               ______________________________________                                        t<100-0-0       1/D1                                                          100-0-0≦t<500-0-0                                                                      2/D1                                                          500-0-0≦t                                                                              3/D1                                                          ______________________________________                                    

The clock message examples correspond to the clock messages described inU.S. Pat. No. 4,837,850. In an outbound clock message, D1 refers to thefirst part of the outbound SOMS signature, this part representing thelevel of the main clock. The figures (100, 500 and MAX) appearing as theconversion limits of the SOMS signature are mere examples and onlyrepresent the mutual magnitude of the figures. The general rule in thisexample is, however, that the value (D1) describing the level of themain clock in the SOMS network is transmitted as such to the systemaccording to the U.S. Patent as the synchronization signature of theclock message.

The conversion examples presented above relate to cases in whichconversion rules are stored fixedly in the node in advance, for instanceat the installation stage of the equipment. The user can also define theinterface between the systems, however. The user thus defines how thesynchronization signature of a message-based system is converted intothe status data of an external system. This is carried out, forinstance, by defining certain class limits, a synchronization signaturebelonging to each class being thus converted into a certainsynchronization status of the external system. For instance, inconnecting SOMS and LP synchronizations, the user is allowed to definehimself or herself the limits at which mcb and lcb bits are transmittedas zero and as one. In FIGS. 5 and 6, the selections defined by the uservia the user interface of the device are denoted by a block 30. In theembodiment of FIG. 5, the settings determined by the user are stored inthe interface unit IU2 (memory location 31) so that the interface unitis able to convert the outbound signature correctly.

The table below shows an example of the interface between a system usingLP synchronization and a SOMS network, it being possible for the user todefine this interface. The user defines the integer values x and yappearing in the table, however in such a manner that x<y<MAX.

    ______________________________________                                        SOMS signature t   mcb bit lcb bit                                            ______________________________________                                        t<x-0-0            0       0                                                  x-0-0≦t<y-0-0                                                                             1       0                                                  y-0-0≦t<MAX-0-0                                                                           0       1                                                  MAX-0-0≦t   1       1                                                  ______________________________________                                    

If the external system is the system described in the above-mentionedU.S. Pat. No. 4,837,850, the conversion can be carried out, forinstance, in the following manner. The user defines the values x and yappearing in the table in such a manner that x<y.

    ______________________________________                                        SOMS signature t                                                                              Clock message T                                               ______________________________________                                        t<x-0-0         1/D1                                                          x-0-0≦tl<y-0-0                                                                         2/D1                                                          y-0-0≦t  3/D1                                                          ______________________________________                                    

Also in this case, D1 refers to the first part of the outbound SOMSsignature.

If the external system is also a SOMS system, the conversion can becarried out, for instance, in the manner shown in the table below. Inthis case, the user defines the values x, y, a, b and c (x<y and a<b<c).In the example, it is assumed that the middle parameter D2 representingthe quality of the synchronization connection in the two different SOMSsystems relates to a different quality parameter, in both of which thevalue zero represents the best possible state with respect to theparameter.

    ______________________________________                                        SOMS signature t1                                                                             SOMS signature t2                                             ______________________________________                                        t1<x-0-0        a-0-D3                                                        x-0-0≦t1<y-0-0                                                                         b-0-D3                                                        y-0-0≦t1 c-0-D3                                                        ______________________________________                                    

D3 is (according to what has been shown above) the last (third) part ofthe outbound SOMS signature.

The SOMS networks connected in the manner described above have theadvantage over one (larger) SOMS network that by means of the interfaceof the invention, it is possible to filter the changes that areinsignificant as regards synchronization and to take into account onlythose changes occurring in the message-based system which aresignificant to synchronization. In this manner, it is possible to removeunnecessary message traffic from the network.

In general, it can be stated that the interface that can be defined bythe user in the manner described above enables a flexible connectionbetween systems, because the user can define the class limits describedabove, whereby the synchronization hierarchy of different systems can beestablished in the desired manner.

Although the invention has been described above with reference to theexamples according to the accompanying drawings, it will be apparentthat the invention is not so restricted but it can be modified withinthe scope of the inventive idea presented above and in the appendedclaims. A more accurate implementation will thus largely depend on thetype of the networks concerned. Since the conversion can also be carriedout on the basis of the message-based synchronization signature receivedfrom the selected source of timing, the synchronization signaturetransmitted by an interface node, this signature being mentioned as thebasic data of conversion in the appended claims, must be understood in abroader sense as any data having a certain relationship with thissignature, the data describing the state of synchronization in the nodeconcerned.

We claim:
 1. A method for connecting a system utilizing message-basedsynchronization and including a plurality of nodes interchanging signalscontaining synchronization messages with information about thepriorities of the respective signals in an internal synchronizationhierarchy of the system utilizing message-based synchronization, with anexternal system having an internal format which has an internalsynchronization hierarchy characterized by having plural levels saidmethod comprising:connecting the external system to the system utilizingmessage-based synchronization by transmitting synchronization statusdata in the internal format of the external system from an interfacenode of the system utilizing message-based synchronization to theexternal system; converting said synchronization status data from amessage-based synchronization signature indicating a level, astransmitted by the interface node, in such a manner that the level inthe internal synchronization hierarchy of the external system asindicated by the synchronization status transmitted to the externalsystem rises or falls corresponding to the rise and, correspondingly,fall of a predetermined magnitude of the level indicated by themessage-based synchronization signature.
 2. A method according to claim1 further including:causing said connecting to take place automaticallyby using conversion rules stored fixedly in said system usingmessage-based synchronization.
 3. A method according to claim 1, furtherincluding causing said converting to take place by using a user of thesystem using message-based synchronization.
 4. A method according toclaim 1, comprising:carrying out said converting in a respective signalinterface of the respective said interface node from whichsynchronization status data is transmitted to the external system.
 5. Amethod according to claim 1, comprising:carrying out said converting ina centralized manner in a decision-making part of the respective saidinterface node.
 6. A method according to claim 1 wherein:said externalsystem is one utilizing LP synchronization and said system utilizingmessage-based synchronization is a SOMS network and wherein saidconnecting includes determining four threshold values for the best SOMSsignature arriving in the respective said node, said threshold valuesdefining four areas for said signature, each of which corresponds to oneof the four bit combinations of the mcb and lcb bits of said LPsynchronization.
 7. A method according to claim 1wherein said systemutilizing message-based synchronization is a SOMS network, and whereinsaid connecting includes transmitting a value describing the level ofthe main clock in the SOMS network, as such, to said external system asthe synchronization signature of a clock message.
 8. A networkarrangement, comprising:a message-based system utilizing message-basedsynchronization, said message-based system comprising a plurality ofnodes interchanging signals containing synchronization messages withinformation about the priority of the respective signals in an internalsynchronization hierarchy of said message-based system; said nodesincluding at least one interface node comprising:converting means forconverting a message-based synchronization signature transmitted by saidinterface node into synchronization status data in the internal formatto an external system in such a manner that the level in the internalsynchronization hierarchy of the external system as indicated by thesynchronization status transmitted to the external system rises or fallscorresponding to the rise and, correspondingly, fall of a predeterminedmagnitude of the level indicated by the message-based synchronizationsignature to obtain a conversion result; transmitting means fortransmitting the conversion result to the external system.